The invention pertains to controlled-source electromagnetic surveying. In particular, this invention pertains to methods and apparatus for acquiring and processing controlled source electromagnetic survey data to identify subsurface features, such as hydrocarbon reserves and other similar subsurface features, that may be characterized in terms of subsurface resistivity data.
For many years, various techniques have been used to identify and monitor hydrocarbon reserves (e.g., petroleum and natural gas) located beneath the earth, both on land and underwater. For example, FIG. 1 illustrates a simplified cross-sectional view of a portion of the earth located below a body of water 10, such as an ocean. Beneath the ocean floor 12 there may be one or more layers of sediment 14, with an oil reservoir 16 buried deep within the sediment 14. The electrical resistivity r1 of sea water 10 is typically much less than the resistivity r2 of sediment 14, which in turn is typically much less than the resistivity r3 of oil reservoir 16. Thus, one way to distinguish between the various subsurface geophysical features involves measuring the electrical resistivity at various subsurface depths, and then using the measured data to create an “image” of the subsurface features. Because the various geophysical features may be located very far below the earth's surface or the sea floor, it is impractical to drill into the earth to directly measure the resistivity of each feature. As a result, various techniques have been developed for measuring the resistivity of subsurface features using equipment located at or above the earth's surface.
One commonly used technique to perform such measurements is called controlled source electromagnetic (“CSEM”) surveying. For example, FIGS. 2 and 3 illustrate a previously known CSEM surveying system 20 that includes a data acquisition module 22 and data processing module 24. Data acquisition module 22 includes one or more transmitters 26 and one or more receivers 28. Each receiver 28 includes a sensor 30, such as a dipole antenna, and a receiver electronics module 32. Receivers 28 may be arranged in a specific configuration relative to transmitter 26. For example, as shown in FIG. 3, receivers 28 may be disposed in a linear array on either side of transmitter 26, with a predetermined spacing D1 between adjacent receivers 28.
Transmitter 26 transmits an electromagnetic signal 34 (e.g., an electric current or magnetic field) into the earth below ocean floor 12, and the sensor 30 in each receiver 28 measures a corresponding received signal 36 (e.g., a voltage and/or magnetic field). Each receiver electronics module 32 includes circuitry used to filter, amplify, and convert received signals 36 to digital data 38 that may be stored for subsequent data processing. Data processing module 24 includes modeling module 40 which uses digital data 38 from receivers 28 to generate a model that may be used to estimate resistivities at various locations (e.g., x1 and x2) in the vicinity of transmitter 26 and receivers 28.
FIG. 4A illustrates an exemplary transmitted signal 34, which may be a bipolar square wave current signal having a peak magnitude I1, and a 50% duty cycle. FIG. 4B illustrates an exemplary received signal 36, which may be a voltage signal that includes a transient component 42 and a steady-state (or quasi-steady-state) component 44. Transient component 42 has a magnitude that typically is much smaller than the magnitude of steady-state component 44. For example, transient component 42 may have a magnitude on the order of about 10−10 volts, whereas steady state component 44 may have a magnitude on the order of about 1 volt.
Although transient data and steady-state data each may be used to estimate resistivities of subsurface structures, prior art CSEM systems typically generate resistivity models using only one data type. Indeed, because receiver electronics modules 32 in prior art CSEM systems typically have limited dynamic range and bandwidth, such systems generally are incapable of detecting both transient component 42 and steady-state component 44 of received signals 36. Thus, many prior art CSEM systems effectively “discard” data that might otherwise be used to generate models of subsurface features.
Further, although some researchers have developed models that incorporate both transient data and steady-state data, such as the system described in A. P. Raiche et al., “The Joint Use Of Coincident Loop Transient Electromagnetic And Schlumberger Sounding To Resolve Layered Structures,” Geophysics 50:1618-1627 (October 1985), such studies typically have used two separate data acquisition systems—a first system that acquires transient data, and a second system that acquires steady state data. Such “dual data” systems are more costly to use than systems that include a single data acquisition system. In particular, prior art dual data systems require configuration, calibration and maintenance of two separate sets of electronics equipment. Further, the time required to perform data collection operations using separate data acquisition systems typically is longer than the time required to gather data using a single data acquisition system. The costs associated with the additional measurement time can be prohibitively expensive, particularly for underwater exploration. In addition to the extra equipment and operational costs associated with prior art dual data CSEM systems, such systems typically have been used for relatively simple, one-dimensional resistivity models, and have only been used to process on-shore data.
It therefore would be desirable to provide methods and apparatus for controlled source electromagnetic surveying that use a single acquisition system to acquire both transient and steady state data either on-shore or off-shore, and jointly process the data to develop one-dimensional, two-dimensional or three-dimensional models of subsurface geophysical structures.